1. Field of the Invention
The present invention relates to a surface acoustic wave filter used as, for example, a band pass filter in a portable telephone, and more particularly, to a longitudinally connected resonator type surface acoustic wave filter.
2. Description of the Related Art
In recent years, advances have been made in the reduction in size and weight of portable telephones. Therefore, a reduction in the number of components defining a portable telephone and miniaturization of the components are required. To meet these requirements, development of components combining a plurality of functions is also progressing.
Surface acoustic filters used at the RF(radio frequency) stage of portable telephones provided with a balanced-unbalanced conversion function, or a balun function, have been developed, and are in use as GSM-type portable telephones.
Such surface acoustic wave filters having a balanced-unbalanced conversion function are disclosed in, for example, Japanese Unexamined Patent Application publication Nos. 6-204781 and 11-97966.
FIG. 22 is a schematic plan view showing the electrode structure of a conventional surface acoustic wave filter having a balanced-unbalanced conversion function.
In this surface acoustic wave filter 100, longitudinally connected resonator type surface acoustic wave filters 101 and 102 are used. These longitudinally connected resonator type surface acoustic wave filters 101 and 102 include three IDTs 101a to 101c, and 102a to 102c, respectively, and reflectors 101d and 101e, and 102d and 102e, respectively.
One-side of the IDTs 101a and 101c of the longitudinally connected resonator type surface acoustic wave filter 101 are commonly connected to an unbalanced signal terminal 104. Likewise, one-side of the IDTs 102a and 102c of the longitudinally connected resonator type surface acoustic wave filter 102 are commonly connected to an unbalanced signal terminal 104.
The central IDTs 101b and 102b of the surface acoustic filters 101 and 102 are connected to balanced signal terminals 105 and 106, respectively.
The phase of the IDT 101b is opposite to that of IDT 102b. Hence, the phases of signals outputted from the terminal 105 and 106 are different from each other by approximately 180xc2x0. Thereby, unbalanced signals input from the terminal 104 are transformed into balanced signals, and are output from the terminals 105 and 106.
FIG. 23 is a schematic plan view showing the electrode structure of the surface acoustic wave filter disclosed in Japanese Unexamined Patent Application Publication No. 06-204781. In this surface acoustic wave filter 200, three IDTs 200a to 200c are arranged in the propagation direction of a surface acoustic wave, and reflectors 200d and 200e are disposed on opposite sides of the area where these IDTs 200a to 200c are arranged. The phase of the IDT 200a is opposite to that of the IDT 200c, and thereby the phases of signals output from the terminals 202 and 203 connected to the respective IDTs 200a and 200c are different from each other by approximately 180xc2x0. Therefore, unbalanced signals input from an unbalanced terminal 201 connected to the IDT 200b are transformed into balanced signals, and are output from the terminals 202 and 203.
FIG. 24 is a schematic plan view showing the electrode structure of the surface acoustic wave filter disclosed in Japanese Unexamined Patent Application Publication No. 11-97966. In this surface acoustic wave filter 300, IDTs 300a to 300c are arranged in order along the propagation direction of a surface acoustic wave. Also, reflectors 300d and 300e are disposed on opposite sides of the area where these IDTs 300a to 300c are arranged.
Herein, one-side end of the IDTs 300a and 300c are commonly connected to an unbalanced signal terminal 301.
On the other hand, one-side comb electrode of the central IDT 300b is divided into comb electrodes 300b1 and 300b2, and these comb electrodes 300b1 and 300b2 are connected to terminals 302 and 303, respectively.
In the surface acoustic wave filter 300, the phase of the IDT 300c is opposite to that of the IDT 300a. Hence, the phases of signals output from the terminal 302 and 303 are different from each other by approximately 180xc2x0. Thereby, unbalanced signals input from the terminal 301 are output from the terminals 302 and 303 as balanced signals.
In any of the above-described surface acoustic wave filters 100, 200, and 300, the output impedance is about four times greater than the input impedance. In these surface acoustic wave filters 100, 200, and 300, when switching the positions of the input terminal and the output terminal, the input impedance becomes four times greater than the output impedance. As a result, a filter providing a balanced-unbalanced output is produced.
A filter having a balanced-unbalanced conversion function requires the transmission characteristics in the pass band between an unbalanced signal terminal and one of the balanced terminals to be equal in the amplitude characteristics to, and different in the phase by 180xc2x0. from, the transmission characteristics between an unbalanced signal terminal and the other of the balanced terminals. These requirements are called xe2x80x9camplitude balance degreexe2x80x9d and xe2x80x9cphase balance degreexe2x80x9d, respectively.
Where a filter device having the above-described balanced-unbalanced conversion function is a three-port device, letting an unbalanced input terminal be port 1, and letting two balanced output terminals be each ports 2 and 3, the amplitude balance degree and the phase balance degree is defined as follows:
Amplitude balance degree=|A|, A=|20xc2x7log(S21)|xe2x88x92|20xc2x7log(S31)|
Phase balance degree=|Bxe2x88x92180|, B=|∠S21xe2x88x92∠S31|
Ideal values of the amplitude balance degree and the phase balance degree in the pass band of the filter, are considered to be 0 dB for the amplitude balance degree, and 0xc2x0 for the phase balance degree. The current market demand for the amplitude balance degree is about 2.0 dB, and that for the phase amplitude balancing is about 20xc2x0.
In reality, however, in any of the surface acoustic wave filters 100, 200, and 300, deviations exist in the above-described balance degrees, and therefore the balance degrees are not sufficient for practical use.
This is because, in the surface acoustic wave filter 100, the electrode fingers of the IDT 101b adjacent to the IDTs 101a and 101c define a ground electrode, whereas the electrode fingers of the IDT 102b adjacent to the IDTs 102a and 102c define a signal electrode, thereby causing significant differences in the frequency characteristics between the surface acoustic wave filter 101 and 102.
FIG. 25 shows the differences in the frequency characteristics between the surface acoustic wave filters 101 and 102 in the surface acoustic wave filter 100 shown in FIG. 22. In FIG. 25, the solid lines show the frequency characteristics of the surface acoustic wave filters 101, and the broken lines show those of the surface acoustic wave filters 102. In any of the surface acoustic wave filters 101 and 102, impedance matching has been achieved with 100 xcexa9. In the figure, the right side scale of the vertical axis shows enlarged frequency characteristics.
As can be seen from FIG. 25, the frequency characteristics of the surface acoustic wave filters 101 and 102 differ greatly. Particularly on the higher frequency side of the pass band range, a significant difference is observed. This difference is a large factor contributing to the deterioration of the above-described balance degrees, when a surface acoustic wave device having a balanced-unbalanced conversion function is made using the surface acoustic wave 101 and 102.
Also, in each of the surface acoustic wave filters 200 and 300, since the polarities of the IDTs on the left and right sides adjacent to the central IDT are different from each other, a difference in the frequency characteristics exists between the pair of balanced signal terminals, and the balance degrees deteriorate just as in the case of the surface acoustic wave filters 100.
To overcome the problems described above, preferred embodiments of the present invention provide a longitudinally connected resonator type surface acoustic wave filter having a balanced-unbalanced conversion function, a fourfold increase in input/output impedance, and greatly improved balance degrees between balanced terminals.
In accordance with a first preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first and second longitudinally connected resonator type surface acoustic wave filters each having a plurality of IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the first longitudinally connected resonator type surface acoustic wave filter has a transmission phase that is substantially opposite that of the second longitudinally connected resonator type surface acoustic wave filter, wherein first terminals of the first and second longitudinally connected resonator type surface acoustic wave filters are each configured as unbalanced terminals by being connected in parallel with each other, and second terminals of the first and second longitudinally connected resonator type surface acoustic wave filters are each configured as balanced terminals via a ground or by being connected in series with each other, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein, in the first and second longitudinally connected resonator type surface acoustic wave filters, each of the plurality of IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein the duty of the electrode fingers in the narrow-pitch electrode finger portion is different between the first and second longitudinally connected resonator type surface acoustic wave filters.
In accordance with a second preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first and second longitudinally connected resonator type surface acoustic wave filters each having a plurality of IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the first longitudinally connected resonator type surface acoustic wave filter has a transmission phase that is substantially opposite to that of the second longitudinally connected resonator type surface acoustic wave filter, wherein first terminals of the first and second longitudinally connected resonator type surface acoustic wave filters are each configured as unbalanced terminals by being connected in parallel with each other, and second terminals of the first and second longitudinally connected resonator type surface acoustic wave filters are each configured as balanced terminals via a ground or by being connected in series with each other, whereby the longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein, in the first and second longitudinally connected resonator type surface acoustic wave filters, each of the plurality of IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein the pitch of the electrode fingers in the narrow-pitch electrode finger portion is different between the first and second longitudinally connected resonator type surface acoustic wave filters.
In accordance with a third preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first and second longitudinally connected resonator type surface acoustic wave filters each having a plurality of IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the first longitudinally connected resonator type surface acoustic wave filter has a transmission phase that is substantially opposite to that of the second longitudinally connected resonator type surface acoustic wave filter, wherein first terminals of the first and second longitudinally connected resonator type surface acoustic wave filters are each configured as unbalanced terminals by being connected in parallel with each other, and second terminals of the first and second longitudinally connected resonator type surface acoustic wave filters are each configured as balanced terminals via a ground or by being connected in series with each other, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein, in the first and second longitudinally connected resonator type surface acoustic wave filters, each of the plurality of IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein the distance between the central axes two adjacent electrode fingers is different between the first and second longitudinally connected resonator type surface acoustic wave filters, at least at one location.
In accordance with a fourth preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first and second longitudinally connected resonator type surface acoustic wave filters each having a plurality of IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the first longitudinally connected resonator type surface acoustic wave filter has a transmission phase that is substantially opposite of the second longitudinally connected resonator type surface acoustic wave filter, wherein first terminals of the first and second longitudinally connected resonator type surface acoustic wave filters are each configured as unbalanced terminals by being connected in parallel with each other, and second terminals of the first and second longitudinally connected resonator type surface acoustic wave filters are each configured as balanced terminals via a ground or by being connected in series with each other, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein, in the first and second longitudinally connected resonator type surface acoustic wave filters, each of the plurality of IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein the distance between the central axes of two adjacent electrode fingers between adjacent IDTs, and/or the distance between the central axes of two adjacent electrode fingers between the narrow-pitch electrode finger portion and the remaining electrode finger portion, are different between the first and second longitudinally connected resonator type surface acoustic wave filters.
In accordance with a fifth preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first and second longitudinally connected resonator type surface acoustic wave filters each having a plurality of IDTs which are successively provided on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the first longitudinally connected resonator type surface acoustic wave filter has a transmission phase that is substantially opposite to that of the second longitudinally connected resonator type surface acoustic wave filter, wherein first terminals of the first and second longitudinally connected resonator type surface acoustic wave filters are each configured as unbalanced terminals by being connected in parallel with each other, and second terminals of the first and second longitudinally connected resonator type surface acoustic wave filters are each configured as balanced terminals via a ground or by being connected in series with each other, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein, in the first and second longitudinally connected resonator type surface acoustic wave filters, each of the plurality of IDTs has a narrow-pitch electrode finger portion in which the pitch on one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein the longitudinally connected resonator type surface acoustic wave filter further includes at least two of the following features (a) to (d):
(a) the duty of the electrode fingers in the narrow-pitch electrode finger portion is different between the first and second longitudinally connected resonator type surface acoustic wave filters;
(b) the pitch of the electrode fingers in the narrow-pitch electrode finger portion is different between the first and second longitudinally connected resonator type surface acoustic wave filters;
(c) the distance between central axes of two adjacent electrode fingers is different between the first and second longitudinally connected resonator type surface acoustic wave filters, at at least one location; and
(d) the distance between central axes of the two adjacent electrode fingers between adjacent IDTs, and/or the distance between central axes of the two adjacent electrode fingers between the narrow-pitch electrode finger portion and the remaining electrode finger portion, are different between the first and second longitudinally connected resonator type surface acoustic wave filters.
In accordance with a sixth preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first and second longitudinally connected resonator type surface acoustic wave filters each having a plurality of IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the first longitudinally connected resonator type surface acoustic wave filter has a transmission phase that is substantially opposite to that of the second longitudinally connected resonator type surface acoustic wave filter, wherein first terminals of the first and second longitudinally connected resonator type surface acoustic wave filters are each configured as unbalanced terminals by being connected in parallel with each other, and second terminals of the first and second longitudinally connected resonator type surface acoustic wave filters are each configured as balanced terminals via a ground or by being connected in series with each other, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function wherein, in the first and second longitudinally connected resonator type surface acoustic wave filters, each of the plurality of IDTs has a chirp type electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is linearly changed along the propagation direction of a surface acoustic wave, and wherein the configuration of the chirp type electrode finger portion is different between the first and second longitudinally connected resonator type surface acoustic wave filters.
In accordance with a seventh preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first to third IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the first and third IDTs have an opposite phase to that of the second IDT, and an unbalanced terminal extends from the second IDT, while an balanced terminal extends from each of the first and third IDTs, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein each of the IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein the duty of the electrode fingers in the narrow-pitch electrode finger portion is different between the portion where the first and second IDTs are adjacent to each other, and the portion where the second and third IDTs are adjacent to each other.
In accordance with an eighth preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first to third IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the first and third IDTs have an opposite phase to that of the second IDT, and an unbalanced terminal extends from the second IDT, while a balanced terminals extends from each of the first and third IDTs, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein each of the IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein the pitch of the electrode fingers in the narrow-pitch electrode finger portion is different between the portion where the first and second IDTs are adjacent to each other, and the portion where the second and third IDTs are adjacent to each other.
In accordance with a ninth preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first to third IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the first and third IDTs have an opposite phase to that of the second IDT, and an unbalanced terminal extends from the second IDT, while a balanced terminal extends from each of the first and third IDTs, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein each of the IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein denoting the middle point of the second IDT as a center, the distance of two adjacent electrode fingers is different between the opposite sides of the center, at least at one location.
In accordance with a tenth preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first to third IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the first and third IDTs have an opposite phase to that of the second IDT, and an unbalanced terminal extends from the second IDT, while a balanced terminal extends from each of the first and third IDTs, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function; wherein each of the IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein the distance between the central axes of the two adjacent electrode fingers between adjacent IDTs, and/or the distance between central axes of the two adjacent electrode fingers between the narrow-pitch electrode finger portion and the electrode finger portion other than the narrow-pitch electrode finger portion, are different between the opposite sides of the second IDT.
In accordance with an eleventh preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first to third IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the first and third IDTs have an opposite phase to that of the second IDT, and an unbalanced terminal extends from the second IDT, while a balanced terminal extends from each of the first and third IDTs, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein each of the IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein the longitudinally connected resonator type surface acoustic wave filter further including at least two of the following features (a) to (d):
(a) the duty of the electrode fingers in the narrow-pitch electrode finger portion is different between the portion where the first and second IDTs are adjacent to each other, and the portion where the second and third IDTs are adjacent to each other;
(b) the pitch of the electrode fingers in the narrow-pitch electrode finger portion is different between the portion where the first and second IDTs are adjacent to each other, and the portion where the second and third IDTs are adjacent to each other;
(c) denoting the middle point of the second IDT be a center, the distance of two adjacent electrode fingers is different between the opposite sides of the above-described center, at least at one location; and
(d) at least one of the distance between central axes of the two adjacent electrode fingers between adjacent IDTs, and the distance between the central axes of the two adjacent electrode fingers between the narrow-pitch electrode finger portion and the electrode finger portion other than the narrow-pitch electrode finger portion, is different between the opposite sides of the second IDT.
In accordance with a twelfth preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first to third IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the first and third IDTs have an opposite phase to that of the second IDT, and an unbalanced terminal extends from the second IDT, while a balanced terminal extends from each of the first and third IDTs, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein each of the IDTs has a chirp type electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is linearly changed along the propagation direction of a surface acoustic wave, and wherein the configuration of the chirp type electrode finger portion is different between the portion where the first and second IDTs are adjacent to each other, and the portion where the second and third IDTs are adjacent to each other.
In accordance with a thirteenth preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first to third IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the second IDT is divided into two portions, the first and third IDTs have an opposite phase to that of the second IDT, and an unbalanced terminal extends from each of the first and third IDTs, while a pair of balanced terminals extend from the second IDT which has been divided into two portions, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein each of the IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein the duty of the electrode fingers in the narrow-pitch electrode finger portion is different between the portion where the first and second IDTs are adjacent to each other, and the portion where the second and third IDTs are adjacent to each other.
In accordance with a fourteenth preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first to third IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the second IDT is divided into two portions, the first and third IDTs have an opposite phase to that of the second IDT, and an unbalanced terminal extends from each of the first and third IDTs, while a pair of balanced terminals extend from the second IDT which has been divided into two portions, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein each of the IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein the pitch of the electrode fingers in the narrow-pitch electrode finger portion is different between the portion where the first and second IDTs are adjacent to each other, and the portion where the second and third IDTs are adjacent to each other.
In accordance with a fifteenth preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter, includes first to third IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the second IDT is divided into two portions, the first and third IDTs have an opposite phase to that of the second IDT, and an unbalanced terminal extends from each of the first and third IDTs, while a pair of balanced terminals extend from the second IDT which has been divided into two portions, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein each of the IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein denoting the middle point of the second IDT be a center, the distance of two adjacent electrode fingers is different between the opposite sides of the above-described center, at least at one location.
In accordance with a sixteenth preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first to third IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the second IDT is divided into two portions, the first and third IDTs have an opposite phase to that of the second IDT, and an unbalanced terminal extends from each of the first and third IDTs, while a pair of balanced terminals extend from the second IDT which has been divided into two portions, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein each of the IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is narrower than that of the other electrode finger portion of the IDT, and wherein at least one of the distance between the central exes of the two adjacent electrode fingers between adjacent IDTs and the distance between the central axes of the two adjacent electrode fingers between the narrow-pitch electrode finger portion and the electrode finger portion other than the narrow-pitch electrode finger portion, is different between the opposite sides of the second IDT.
In accordance with a seventeenth preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first to third IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the second IDT is divided into two portions, the first and third IDTs have an opposite phase to that of the second IDT, and an unbalanced terminal extends from each of the first and third IDTs, while a pair of balanced terminals extend from the second IDT which has been divided into two portions, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function, wherein each of the IDTs has a narrow-pitch electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of the each IDT is narrower than that of the other electrode finger portion of the IDT, and wherein the longitudinally connected resonator type surface acoustic wave filter further includes at least two of the following features (a) to (d):
(a) the duty of the electrode fingers in the narrow-pitch electrode finger portion is different between the portion where the first and second IDTs are adjacent to each other, and the portion between the second and third IDTs are adjacent to each other;
(b) the pitch of the electrode fingers in the narrow-pitch electrode finger portion is different between the portion where the first and second IDTs are adjacent to each other, and the portion where the second and third IDTs are adjacent to each other;
(c) denoting the middle point of the second IDT be a center, the distance of two adjacent two electrode fingers is different between the opposite sides of the above-described center, at least at one location;
(d) at least one of the distance between the central axes of the two adjacent electrode fingers between adjacent IDTs and the distance between the central axes of the two adjacent electrode fingers between the narrow-pitch electrode finger portion and the electrode finger portion other than the narrow-pitch electrode finger portion, is different between the opposite sides of the second IDT.
In accordance with an eighteenth preferred embodiment of the present invention, a longitudinally connected resonator type surface acoustic wave filter includes first to third IDTs which are successively arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, wherein the second IDT is divided into two portions, the first and third IDTs have an opposite phase to that of the second IDT, and an unbalanced terminal extends from each of the first and third IDTs, while a pair of balanced terminals extend from the second IDT which has been divided into two portions, whereby this longitudinally connected resonator type surface acoustic wave filter has a balanced-unbalanced conversion function; and wherein each of the IDTs has a chirp type electrode finger portion in which the pitch of one electrode finger portion from the adjacent IDT-side end of each of the IDTs is linearly changed along the propagation direction of a surface acoustic wave; and wherein the configuration of the chirp type electrode finger portion is different between the portion where the first and second IDTs are adjacent to each other, and the portion where the second and third IDTs are adjacent to each other.
Furthermore, another preferred embodiment of the present invention provides a communication device including a longitudinally connected resonator type surface acoustic wave filter in accordance with any one of the preferred embodiments described above.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.